Tentacles of destruction

5042012

This last post before Easter is something I’ve thought more and more about over the last few years. I wouldn’t have given it much time in the past, but I’m now convinced roads are one of the humanity’s most destructive devices. Let me explain.

Before I had a good grasp of extinction dynamics, I wouldn’t have attributed much import to the role of roads in conservation. I mean, really, a little road here and there (ok, even a major motorway) couldn’t possibly be a problem? It’s mostly habitat destruction itself, right?

Not exactly. With our work on extinction synergies, I eventually came to realise that roads are some of the first portals to the devastation to come.

Figure 1. (A) A large population within unmodified, contiguous habitat fluctuates near full carrying capacity (K). (B) When habitat is reduced (e.g. 50% area loss), total abundance declines accordingly. (C) All remaining fragmented subpopulations have limited connectivity, implying much greater extinction risk than that predicted for the same habitat loss in less fragmented landscapes.

It sort of works like this (see Fig. 1 from our paper in Trends in Ecology and Evolution). Instead of destroying habitats (forests, reefs, grasslands, savannas, etc.) systematically (i.e., in large, contiguous blocks – Fig. 1B), we tend to fragment landscapes into many disconnected patches (Fig 1C). The vehicle by which we do this in terrestrial (especially forest) ecosystems is with roads connecting the matrix. As in turns out, instead of reducing biodiversity by the proportion of primary habitat lost (in this case, by 50 %), the multi-patch type of fragmentation leads to a much larger proportional biodiversity loss. This is because of the interactive effects of piecemeal fragmentation: roads between patches facilitate human and other predator access to areas previously isolated, micro-climatic changes resulting from a high proportion of ‘edge’ habitats can alter suitability, and invasive species have many more pathways by which they can spread.

The authors talk about so-called ‘ecopassages’, which have been put in place in many road developments to mitigate wildlife ‘conflict’, such as crossing structures (bridges, underpasses, etc.) that are supposed to link habitats bisected by roads too difficult to cross safely for most species. Their main point is, however, that most of these lack any real scientific rigour in either testing their effects (do they actually work?), or even planning them from the outset. The authors go on to give a detailed description about how road developments should design transportation networks with wildlife connectivity in mind prior to building, and that rigorous scientific protocols should be developed to test their effectiveness. Fair cop.

However true these pleas and recommendations are, I still think the paper misses the main point (and it has a decidedly North American bias) – why do we need so many roads in the first place? Indeed, in most places in the world now, finding primary habitats is difficult enough, so surely we can be a bit cleverer about putting fewer roads in places where they are not strictly needed?

The point is that roads are generally very bad, not just because they impede connectivity, but because they open up a Pandora’s box of conservation nightmares still to come. Still, forcing road-network planners to consider these issues pre- and post-development can only do good.

Great post and thanks for the perspective on an important issue. Do you have any thoughts and examples of studies that come to mind that also quantitatively or qualitatively link road access to changing land-use patterns by hunters and the subsequent impacts of harvest and access on wildlife populations?